Marc Antony wasn't exactly mediocre to ok, he was pretty damn good. But his arrogance and numerous flaws destroyed him one by one, as well as his attempt to invade Parthia, which was essentially the "Russia in the Winter" meme of the First Century BC. He would never have lost to Augustus himself in a straight fight, and probably could've bested Agrippa, but he never got the chance. He lost his support, his popularity, and his troops one by one to the politicking and scheming of Augustus. When time came for the final battle, what few men he had left were slipping away to join the enemy in the dead of night.